Running in a 5K race in 2007 after a year of hiatus from training, mind you, I could hardly reach the finish line. Without proper training, a five-kilometer run would't be that easy.
Recently I came across two websites that discuss how the 5km races help one’s fitness and motivate runners to train and improve their personal best or record (PB/PR). Here are the links:
3 Reasons to Run a 5K
Five-kilometer runs boost for fitness, charities